This invention relates in general to devices which emit radiation, and in particular to a device which improves the coherence of radiation from a partially coherent source, particularly an x-ray source. It is currently difficult and expensive to generate coherent x-rays using laser techniques. Accordingly, when coherent x-rays are required for interference experiments and the like, it is typical to employ a source of partially coherent x-rays followed by slits which filter out non-coherent portions of the beam. But this technique for improving coherence has a disadvantage: the slits permit only a fraction of the total beam energy to pass to the exit port. This inefficiency results in longer experiment times, and increased expense, and when the signal to noise ratio is too low, it may inhibit the experiment altogether.
In the process of increasing the transverse coherence of a radiation beam pulse by the method of this invention, the emittance of the beam is being reduced along at least one transverse direction. "Transverse direction" is defined here as one which is perpendicular to the direction of propagation of the beam. That reduction can be useful even if the beam is not to be used for interference experiments, for example, when it is desired to compress the length of the original beam pulse, i.e. to have substantially all of the radiation impinge on a surface during a time, .delta.t, shorter than the original time duration of the pulse. That can be accomplished by dynamical optical means and the smallest achievable .delta.t depends on the transverse emittance of the beam pulse. A reduction of the transverse emittance along at least one direction will, therefore, allow one to achieve smaller .delta.t values.